The present invention relates generally to a nuclear reactor core and fuel elements therefor, and more particularly to a nuclear reactor core for a high temperature gas-cooled reactor (HTGR) comprising stackable fuel elements.
In order that spent nuclear fuel may be cyclically replaced, a nuclear reactor core may be constituted by fuel elements comprising stackable blocks having fuel chambers. Generally, such fuel elements have passageways so that coolant gas can flow through the blocks and columns to cool the core and transfer the generated heat. Stackable fuel elements may be stacked to form fuel columns which may in turn be interfitted side-by-side to constitute the reactor core. One convenient stackable form is a hexagonal prism.
It has been found that in such an assembled core, coolant may escape into interstices between vertically and horizontally adjacent blocks from the entry plenum, bypassing the intended coolant passageways. Additionally, coolant gas can escape from the coolant passageways and into the interstices. The escaped coolant gas could build up pressure haphazardly in the gaps between the blocks. This uneven buildup of pressure could exert mechanical forces sufficient to move the blocks and destabilize the core. The shutdown of the nuclear reactor at Fort St. Varian, Colorado, is partly attributable to such flow instabilities.
One solution to such problems has been to require small interblock clearances and sealing at either the entry or exit plenum faces. The sealing and the closer machining tolerances for the fuel blocks constitutes an additional expense and an impediment to the efficient replacement of fuel elements for refueling purposes.
The designer is faced with compromising among conflicting design objectives. That is, one goal of reactor design is to maximize power output. One factor in determining power output in a stackable core is the fuel capacity of the fuel elements. It is desirable to maximize the volume of the fuel chambers of the individual fuel blocks. However, high fuel and power capacity presents certain complications.
In the first place, high power capacity requires that more heat be transferred from the core so that the generated power can be used and so that the core does not overheat. Cooling and heat transfer may be provided for by means of holes drilled vertically through the fuel elements. The holes align with corresponding holes in vertically adjacent blocks so that coolant passageways extend vertically through the reactor columns. A large cross-sectional area of coolant holes is desirable to maximize the cooling and heat transfer characteristics of the system. In operation, gas flows downwardly through the coolant passageways removing heat from the core.
Furthermore, it should be understood that the fuel element blocks are subjected to considerable stress, and that higher power tends to create greater stress. The fuel blocks are subject to extremes of heat and radioactive bombardment. In the case of heat, the blocks are subjected not only to high maximum temperatures, but also to severe temperature gradients which can induce differential expansion so as to impose mechanical stresses in the blocks themselves. Furthermore, the blocks may be subjected to other mechanical stresses through block movements. Fuel blocks must, therefore, be constructed of highly refractory material such as graphite. Since the block is peppered with fuel chambers and coolant holes, it is necessary to maintain a minimum ligament thickness between such holes and chambers so that the block can withstand the stresses imposed upon it.
It will be apparent from the foregoing that some of the design goals for stackable fuel elements are in conflict. In particular, for a given size block, the goals of maximizing fuel capacity to achieve high power output, maximizing coolant cross section to facilitate heat transfer, and maximizing ligament thickness to ensure mechanical integrity under high stress oppose one another. For example, one cannot maximize the size of the fuel holes and the coolant holes and still maintain adequate ligament thickness between the holes.
Consequently, fuel element design is concerned with determining optimal comprises among fuel capacity, coolant cross section, and ligament thickness. As discussed below, the present invention permits comprises superior to those available under the constraints of the prior art. An arrangement of fuel chambers and coolant holes that effects a good compromise considering the limitations of the art prior to the present invention is taught in Fortescue et al., U.S. Pat. No. 3,413,196, which is incorporated herein as though quoted in full.
Confusion of intercolumn coolant flow and intracolumn coolant flow is another problem addressed by the present invention. The problem is particularly acute with respect to core designs incorporating commonly orificed seven-column zones or "patches". In such a core, each patent is covered by a cap with a variable orifice to control the coolant flow through each patch. Gaps between the caps of adjacent patches permit coolant to flow between the patches. Thus the between-patch flow is often under a different pressure than that of the within-patch flow, thereby contributing to the mechanical instability of the blocks discussed above.
Slight tilting of fuel blocks due to pressure differentials (or possibly surface irregularities in the block) enlarges the interstices between vertically adjacent blocks creating a pressure drop in the region between such blocks. This pressure drop can draw some of the between-patch flow across the horizontal faces of the throttle the coolant flow through the intended coolant passageways impairing the efficiency of the cooling system and adding further uncertainties to the coolant pressure distribution. Moreover, the cross-flow has been known to enter handling holes and flow toward the coolant, thereby reducing coolant efficienty.
These inefficiencies and pressure differentials are not, important so much because of their magnitude, but because of the uncertainity they introduce into the reactor.